Method and apparatus for conveying a cellulosic feedstock

ABSTRACT

A method and apparatus for preparing a cellulosic feedstock are disclosed. Embodiments of the method comprise passing the cellulosic feedstock through an optional impregnation chamber to an outlet of the impregnation chamber, passing the cellulosic feedstock from the outlet of the impregnation chamber to a holding tank having an inlet and an outlet, and conveying the cellulosic feedstock downwardly and laterally as it travels through the holding tank. Embodiments of the apparatus comprise at least one sidewall defining a passage. The passage has an upper portion and a lower portion, and the lower portion has a greater cross-sectional area than the upper portion. At least one inlet is provided adjacent the upper portion, and at least one outlet is provided adjacent the lower portion, at an elevation below the inlet.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims benefit under 35 USC 120 as a divisional application of co-pending U.S. patent application Ser. No. 12/181,724, filed Jul. 29, 2008, which itself claims the benefit of priority under 35 USC 119 from Canadian Patent Application No. 2,638,157, filed Jul. 24, 2008, the specifications of which are incorporated herein by reference.

FIELD

The invention relates to a method and apparatus for preparing a cellulosic feedstock for the subsequent production of a fermentable sugar stream from the cellulose and hemicellulose in the cellulosic feedstock wherein the fermentable sugar stream may be used for subsequent ethanol production. More specifically, the invention relates to a holding tank, and a method of utilizing the holding tank whereby bridging may be reduced or eliminated.

BACKGROUND

Several processes for the production of ethanol are known. Generally, the production of fuel ethanol involves the fermentation of sugars with yeast. Typically, the sugars are derived from grains, such as corn and wheat. The starches in the grains are subjected to enzymatic hydrolysis in order to produce the sugars, which are then subjected to anaerobic fermentation to produce ethanol.

Plant materials are a significant source of fermentable sugars, such as glucose that can be transformed into biofuels. However, the sugars in plant materials are contained in long polymeric chains of cellulose and hemicellulose. Utilizing current fermentation processes, it is necessary to break down these polymeric chains into monomeric sugars, prior to the fermenting step.

Recently, processes have been developed for utilizing cellulosic feedstock, such as corncobs, straw, and sawdust, to produce sugars for ethanol fermentation. Such processes typically comprise pre-treating the feedstock to increase the accessibility of the cellulose to hydrolysis enzymes, and subjecting the cellulose to cellulase enzyme systems to convert the cellulose into glucose.

Methods of converting plant biomass into fermentable sugars are known in the art and in general comprise two main steps: a pre-treatment step to activate the plant structure, and an enzymatic or chemical hydrolysis step to convert the polymeric chains of cellulose and hemicellulose into monomeric sugars. Several approaches have been used for the pre-treatment step, e.g., autohydrolysis, acid hydrolysis, ammonia activation, kraft pulping, organic solvent pulping, hot water pre-treatment, ammonia percolation, lime pre-treatment, caustic soda pulping, or alkali peroxide pre-treatment. Early pre-treatment steps included grinding or milling the feedstock into a powder, which was then mixed with water to form a slurry.

More recently, solvent based pre-treatments, alkali pre-treatments, and acidic pre-treatments have also been described. PCT publication WO/2007/009463 to Holm Christensen describes an alternate pre-treatment, which does not involve the addition of acids, bases, or other chemicals. This pre-treatment process involves soaking the cellulosic material in water, conveying the cellulosic material through a heated and pressurized reactor, and pressing the cellulosic material to produce a fiber fraction and a liquid fraction. After pressing the cellulosic material, the cellulosic material is exposed to hydrolysis enzymes.

Each pre-treatment technology has a different mechanism of action on the plant structure, inducing either physical and/or chemical modifications. However, the main objective of the pre-treatment is to provide accessibility of the plant material to the enzymes.

SUMMARY

The commercial viability of a hydrolysis process is dependent on the character of the feedstock provided to the hydrolysis unit. Typically, this requires that a feedstock is activated such that a significant portion (e.g., greater then 75%) of the cellulose and hemicellulose of the feedstock is accessible to hydrolysis enzymes. If such an activated feedstock is provided to a hydrolysis unit, then at least 60%, preferably more than 75% and more preferably over 90% of the cellulose and hemicelluloses may be converted to monomeric sugars. This sugar rich process stream may subsequently be subjected to fermentation to produce an alcohol stream. The alcohol stream from the fermentation stage (i.e., the raw alcohol stream) may have an ethanol content of about 3-22% v/v, preferably about 5-15% and more preferably more about 8-12%.

An activated feedstock for hydrolysis is preferably prepared by auto hydrolysis, which is preferably conducted in a steam explosion reactor also known as a digester. Autohydrolysis is a process of breaking down hemicellulose and cellulose by exposure to high temperatures, steam and pressure. When performed in the presence of an acid, auto hydrolysis is known as an acid hydrolysis.

During autohydrolysis, the degree of polymerization of cellulose may be reduced from about 10,000 to about 1,500-1,000. This process is preferably carried out above the glass transition temperature of lignin (120-160° C.). Depending upon the severity of the reaction, degradation products may be produced, such as furfural, hydroxyl-methylfurfural, formic acid, levulinic acid and other organic compounds.

During a steam explosion treatment, a lignocellulosic feedstock is subjected to elevated heat (e.g., 180° C. to 220° C.) and pressure (e.g., 131 psig to 322 psig) in the presence of suitable chemicals (e.g., organic/and/or inorganic acids, ammonia, caustic soda, sulfur dioxide, solvents etc) in a pressurized vessel. The treated lignocellulosic feedstock is then released from the pressurized vessel such that the pressure is rapidly reduced (e.g., 1 second or less). The biomass may exit the hydrolyzer into a reduced pressure, preferably atmospheric pressure and, more preferably into a vacuum. The rapid decrease in pressure results in the biomass separating into individual fibres or bundles of fibres. This step opens the fibre structure and increases the surface area. The lignin remains in the fibre along with cellulose and residual hemicellulose. Accordingly, the explosive release of pressure, combined with the high temperature and pressure treatment results in the physicochemical modification of the lignocellulosic feedstock that is then suitable for feeding to a hydrolysis unit.

In order for the steam explosion process to be able to produce an activated feedstock that is capable of producing such a sugar rich process stream, the temperature and moisture level of the cellulosic feedstock that is fed to a steam explosion reactor preferably is relatively uniform and preferably has a temperature from about 50 to about 70° C., and more preferably 50-65° C. and a moisture content from about 30 to about 60 wt % (preferably 45 to about 55 wt %). Moisture content is the quantity of water contained in a material, and on a weight basis, is the weight of water in the material divided by the mass of the material.

Without being limited by theory, it is believed that an unexpected increase in the conversion of the feedstock to fermentable sugars may be achieved if the moisture content of the feedstock fed to the steam explosion reactor is lower, provided that sufficient water is present for hydrolyzing and/or activating the feedstock. If the feedstock is too dry, then there may be insufficient water molecules present in the fiber and hence not all of the feedstock will be activated and/or hydrolyzed (i.e., the hydrolysis reaction/activation will not occur at all possible sites). Accordingly, it might be presumed that a substantial excess of water should be used to ensure water molecules are available at each hydrolysis/activation site. Surprisingly, it has been determined that if the cellulosic feedstock that is fed to a steam explosion reactor has an excess of moisture then a smaller percentage of the available sites of the feedstock are activated/hydrolyzed than would be expected. It is believed that this is due to the high moisture content acting as a barrier to heat transfer through the fiber structure. The external fiber reaches the process temperature far in advance to the internal fiber, hence resulting in very uneven heat transfer and the resulting uneven autohydrolysis reaction. Further, during the autohydrolysis process additional water may be provided to the process by way of direct injected steam in order to raise the fiber temperature from the inlet temperature to the outlet temperature of the reactor. If the inlet moisture content of the fiber is at saturation, then the additional water will be free water in the autohydrolysis reactor resulting in washing of the soluble hemicellulose from the fiber and causing subsequent accumulation of hemicellulose within the reactor, Over time, the accumulated hemicellulose will tend to break down to inhibitor compounds and deposit degraded sugars on the internal components of the reactor. These deposits will become an obstruction to the flow of the biomass.

It has also been determined that if the cellulosic feedstock that is fed to a hydrolyzer has a temperature that is too high, then some percentage of the hemicellulose sugars will be degraded to inhibitory compounds prior to starting the autohydrolysis reaction and further amounts during the autohydrolysis reaction itself. Conversely, if the fiber is too cold entering the hydrolyzer, the first one third to one half of the reactor vessel may act as a preheating device rather than as a hydrolyzer, resulting in incomplete autohydrolysis. Accordingly, it is preferred to have very consistent fiber temperature year round as well as from night to day time operation, for the fiber that is fed to the hydrolyzer.

Alternately, and in addition, it is preferred that the fiber in the feedstock fed to the autohydrolysis unit have a relatively uniform temperature profile. For example, it is preferred that the core of the blocks of material have a temperature that is within 80%, more preferably 90%, most preferably 95% of the temperature of the exterior surface of the material. Accordingly, for example, if the temperature of the exterior surface of the material is from 50 to 70° C., then the temperature of the core of the material is preferably from 45 to 63° C.

It has also been determined that the fiber requires time for the moisture that is added to become equilibrated throughout the entire fiber particle. It has been determined that under laboratory conditions, it may take from 5 to 9 minutes to equilibrate the moisture content of the fiber. Under industrial conditions it will be longer. Preferably, the autohydrolysis reaction time in the vessel is typically about 5 to 6 minutes or less. It is preferred that the fiber in the feedstock fed to the autohydrolysis unit have a relatively uniform moisture profile. For example, it is preferred that the core of the blocks of material have a moisture content that is within 80%, more preferably 90%, most preferably 95% of the moisture content of the exterior surface of the material. Accordingly, for example, if the moisture content of the exterior surface of the material is from 45 to 55 wt %, then the moisture content of the core of the material is preferably from 40.5 to 49.5 wt %.

A feedstock having a moisture content from about 45 to about 55 wt may be prepared by obtaining relatively dry plant material which is broken down into small chips, e.g., from about 0.05 to about 2 inches, and then combining the chip with water (e.g., steam and/or a fine mist spray, such as droplets of water of between 600μ and 900μ in diameter). This material may then be transported to a hydrolysis or auto hydrolysis reactor. This material is difficult to transport as the material is essentially a solid (having insufficient water to form even a slurry). Accordingly, the material has a tendency to interlock and may result in process vessels or flow passages between equipment becoming blocked.

Embodiments of the present invention provide a method and apparatus for transporting a cellulosic feedstock. The method and apparatus relate to a holding tank that can be positioned downstream from a cellulosic feedstock pre-treatment process, and that can be utilized to further prepare the cellulosic feedstock for, e.g., auto hydrolysis or hydrolysis.

In one broad aspect, a method of preparing a cellulosic feedstock for ethanol production is provided. The method comprises passing the cellulosic feedstock through an impregnation chamber to an outlet of the impregnation chamber, passing the cellulosic feedstock from the outlet of the impregnation chamber to a holding tank having an inlet and an outlet, and conveying the cellulosic feedstock downwardly and laterally as it travels through the holding tank.

Embodiments in accordance with this broad aspect may be advantageous because the lateral movement of the feedstock assists in preventing the feedstock from plugging the apparatus. In particular, if the holding tank had walls that extended vertically and defined a vertical passage therethrough, then the feedstock would travel vertically downwardly. Due to the nature of the feedstock, the feedstock has a tendency to bridge the vertically extending passage, resulting in some, or possibly all, of the passage becoming blocked. Bridging of the passage can result in the need for manual intervention, as well as monitoring, to ensure that feed material is supplied to downstream process equipment on a continuous basis. Further, if the holding tank is provided with a steam jacket, overly long holdup of material in the holding tank could result in degradation of some of the cellulose and hemicellulose in the feedstock.

In some embodiments, the method further comprising maintaining a temperature in the holding tank between about 50° Celsius and about 75° Celsius. Such embodiments may be advantageous because the elevated temperature may prepare the feedstock exiting the holding tank to be at a predetermined temperature for the next process stage. Further, if the feedstock entering the holding tank is at the predetermined temperature for the next process stage, the feedstock may be maintained at a desired temperature as it passes through the holding tank.

In some embodiments, the cellulosic feedstock moves from the inlet to the outlet of the holding tank in about 10 to about 30 minutes.

In some embodiments, the inlet of the holding tank is disposed at an elevation above the outlet of the holding tank, and the cellulosic feedstock migrates from the inlet towards the outlet of the holding tank under the force of gravity.

In some embodiments, the holding tank comprises an outlet, and the method further comprises conveying the cellulosic feedstock laterally across the outlet.

In some embodiments, the step of conveying the cellulosic feedstock laterally across the outlet comprises actively withdrawing the cellulosic feedstock from essentially an entirety of the outlet.

In some embodiments, the step of conveying the cellulosic feedstock laterally across the outlet comprises withdrawing a generally equal amount of cellulosic feedstock from each portion of the outlet.

In some embodiments, the step of conveying the cellulosic feedstock laterally across the outlet comprises withdrawing a first portion of the cellulosic feedstock in a first lateral direction and withdrawing a second portion of the cellulosic feedstock in a second lateral direction.

In another broad aspect, a holding tank for a cellulosic feedstock is provided. The holding tank apparatus comprises at least one sidewall defining a passage. The passage has an upper portion and a lower portion, and the lower portion has a greater cross-sectional area than the upper portion. At least one inlet is provided adjacent the upper portion, and at least one outlet is provided adjacent the lower portion, at an elevation below the inlet.

Embodiments in accordance with this broad aspect may be advantageous because providing the lower portion with a greater cross-sectional area than the upper portion may prevent cellulosic material from adhering or sticking to the sidewalls as the cellulosic material passes through the holding tank. Accordingly, each portion of cellulosic feedstock that passes through the holding tank may have essentially the same residence time in the passage. This may be advantageous in helping to ensure that the feedstock exiting the holding tank has a more uniform temperature and/or moisture content.

In some embodiments, the sidewalls comprise a first sidewall and a second sidewall opposed to the first sidewall, and the first and second sidewalls diverge relative to each other from the upper portion to the lower portion.

In some embodiments, the sidewalls comprise a third sidewall and a fourth sidewall opposed to the third sidewall. The third and fourth sidewalls extend between the first and second sidewalls, and the third and fourth sidewalls diverge relative to each other from the upper portion to the lower portion.

In some embodiments, the holding tank apparatus further comprises at least one discharge member adjacent the outlet. The discharge member may serve to withdraw the cellulosic feedstock from the outlet and direct it towards one or more hydrolysis reactors.

In some embodiments, the at least one discharge member comprises a base and an open top that is at least as large as, and is in vertical registration with, the outlet, and at least a first discharge member outlet that is laterally positioned.

In some embodiments, the at least one discharge member comprises at least one screw conveyor mounted above the base.

In some embodiments, the at least one discharge member comprises a first pair of screw conveyors, and each screw conveyor in the first pair rotates about respective first and second generally parallel axes. In some embodiments, the axes extend at an angle of less than 45° relative to horizontal. In further embodiments, the axes are generally horizontal.

In some embodiments, the screw conveyors are rotated in the same direction and feed material from above towards the first discharge member outlet.

In some embodiments, the screw conveyors have a first pitch adjacent the first discharge member outlet, and a second pitch narrower than the first pitch distal to the first discharge member outlet. Such embodiments may be advantageous because a generally equal amount of feedstock may be withdrawn from each portion of the lower end.

In some embodiments, the screw conveyors each have a first flight and a second flight, and the first flight has a first pitch and is disposed adjacent the first discharge member outlet, and the second flight has second pitch narrower than the first pitch and is disposed along the shaft upstream from the first flight. In some further embodiments, each screw conveyor comprises a third flight intermediate the first and second flights, and the third flight has an intermediate pitch that is wider than the first pitch and narrower than the wider pitch.

In some embodiments, the discharge member further comprises a second pair of screw conveyors. In some such embodiments, the screw conveyors of the second pair are rotatable about parallel axes. Preferably, the axes are generally horizontal. Further, each screw conveyor in the second pair preferably rotates in the same direction.

In some embodiments, the second pair of screw conveyors urges material from above towards a second discharge member outlet that is laterally positioned and spaced away from the first discharge member outlet.

In some embodiments, the second discharge member outlet is laterally opposed to the first discharge member outlet. In some further embodiments, the screw conveyors of first pair rotate opposite to the screw conveyors of second pair.

In some embodiments, the holding tank apparatus further comprises a heating jacket provided on at least a portion of the sidewalls. In some embodiments, the heating jacket is provided on the sidewalls.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be more fully and particularly understood in connection with the following description of the preferred embodiments of the invention in which:

FIG. 1 is a perspective illustration of an embodiment of a holding tank of the present invention, showing an impregnation chamber that may be positioned upstream from the holding tank;

FIG. 2 is a perspective illustration of the impregnation chamber shown in FIG. 1;

FIG. 3 is a top cutaway view of the impregnation chamber of FIG. 1;

FIG. 4 is a cross section taken along line 4-4 in FIG. 2;

FIG. 5 is a cross section taken along line 5-5 in FIG. 1;

FIG. 6 is a perspective view of an embodiment of discharge member of the present invention, shown removed from a holding tank;

FIG. 7 is a cross-section taken along line 7-7 in FIG. 6;

FIG. 8 is a top view of the discharge member of FIG. 6;

FIG. 9 is a top view of an alternate embodiment of a discharge member of the present invention; and,

FIG. 10 is a cross-section taken along line 10-10 in FIG. 1.

DETAILED DESCRIPTION

The cellulosic feedstock is preferably a lignocellulosic feedstock. A lignocellulosic feedstock is derived from plant materials. As used herein, a “lignocellulosic feedstock” refers to plant fiber containing cellulose, hemicellulose and lignin. In some embodiments, the feedstock may be derived from trees, preferably deciduous trees such as poplar (e.g., wood chips). Alternately or in addition, the feedstock may also be derived from agricultural residues such as, but not limited to, corn stover, wheat straw, barley straw, rice straw, switchgrass, sorghum, sugarcane, bagasse, rice hulls and/or corn cobs. Preferably, the lignocellulosic feedstock comprises agricultural residues and wood biomass, more preferably wood biomass and most preferably hardwoods. The applicants contemplate other sources of plant materials comprising cellulose, hemicellulose and lignin for use in deriving lignocellulosic feedstocks and any of those may be used.

The lignocellulosic feedstock is preferably cleaned, e.g., to remove ash, silica, metal strapping (e.g., from agricultural products), stones and dirt. The size of the components of the lignocellulosic feedstock may also be reduced. The size of the components of the feedstock may be from about 0.05 to about 2 inches, preferably from about 0.1 to about 1 inch, and more preferably from about 0.125 to about 0.5 inches in length. For example, the cellulosic feedstock may comprise fibers, e.g., chopped straw, of a length of between about 4 mm and about 7 mm. Any process machinery that is able to crush, grind or otherwise decrease the particle size may be utilized.

The feedstock is preferably treated with water as to have a moisture content upon entry to holding tank 100 of from about 30 to about 60 wt %, Same as the other ones preferably from about 45 to about 55 wt %. For example, referring to FIGS. 1 and 2, an embodiment of a holding tank 100 of the present invention is shown wherein the holding tank 100 is positioned downstream from an impregnation chamber 10, which is preferably used to pre-treat the feedstock prior to the feedstock entering holding tank 100. Impregnation chamber 10 is preferably configured to pre-treat the cellulosic feedstock, for example by moistening and/or heating the cellulosic feedstock.

A preferred impregnation chamber 10 is exemplified in FIGS. 2-4, impregnation. As shown therein, impregnation chamber 10 may comprise an inlet 11, one or more conveyance members 12 for urging the cellulosic feedstock along the length of the chamber, one or more moisture injection ports 14, which may be provided on paddles 20 of conveyance member 12 and/or inner wall 22 of impregnation chamber 10, for injecting moisture into the cellulosic feedstock, one or more heating jackets 16 provided outward of inner wall 22 for heating the cellulosic feedstock, and an outlet 18. In order to prevent material stagnating in impregnation chamber 10, impregnation chamber 10 may have a bottom wall 24 that has two or more portions each of which has a conveyance member 12 associated therewith. Bottom wall 24 and conveyance member 12 are preferably configured such that bottom wall 24 is swept as conveyance member 12 rotates. For example, as exemplified in FIG. 4, bottom wall 24 may be scallop shaped, e.g., have two inverted arches or troughs. Further details regarding various embodiments of optional impregnation chamber 10 may be found in co-pending U.S. patent application Ser. No. 12/181,596 filed on Jul. 29, 2008, the disclosure of which is incorporated herein by reference in its entirety.

After the cellulosic feedstock is optionally pre-treated in impregnation chamber 10, it is directed to holding tank 100, where it is held or contained for a residence time, such that, for example, moisture added in impregnation chamber 10 has sufficient time to penetrate into the feedstock so that the feedstock is ready for downstream processing. Alternately, or in addition, the feedstock may require additional time for all portions of the feedstock to be raised to a predetermined temperature that is suitable for downstream processing. Alternately, the feedstock entering holding tank 100 may be at the predetermined conditions for downstream processing and holding tank is used as a reservoir to hold prepared feedstock such that downstream processes may operate on a continuous basis. . From holding tank 100, the cellulosic feedstock may be directed to one or more downstream process units, preferably auto hydrolysis and/or hydrolysis reactors (not shown), such that the cellulose may be hydrolyzed to produce sugars that are suitable for fermentation to ethanol.

As exemplified in FIGS. 1 and 5, holding tank 100 is oriented such that the passage through holding tank 100 extends generally downwardly and the passage therethrough is configured so as to reduce, and preferably essentially prevent, bridging of feedstock in holding tank 100. Accordingly, it is preferred that the passage through holding tank 100 extends generally downwardly and that the passage has a greater cross sectional area at the lower end then the upper end. More preferably, the cross sectional area continually increases in the downward direction. This may be achieved by constructing the passage of the holding tank with one or more walls that diverge in the downward direction.

If the feedstock passing downwardly through holding tank interlock, it may form a blockage by a process known as bridging. The blockage may extend all the way across the passage in holding tank 100 thereby preventing downward movement of feedstock and causing a gap in the supply of feedstock to the downstream process unit. Alternately, it may block only part of the passage. In any event, intervention would then be required to remove the blockage. The interruption of feedstock delivery to the downstream process unit could require part of a plant to be shut down while the blockage is removed thereby reducing throughput and also requiring the plant to be brought back to steady state operating conditions once the blockage is cleared. Accordingly, the holding tank may require monitoring to permit intervention at an early stage should bridging occur. By increasing the cross sectional area in the downstream direction, the tendency of the feedstock to form a blockage of the passage is reduced and may be eliminated.

As exemplified in FIGS. 5 and 10, holding tank 100 comprises at least one sidewall 102, which defines a volume or passage 104. In the embodiment shown, holding tank 100 comprises four sidewalls, namely front wall 102 a and a spaced apart opposed rear wall 102 b, and a side wall 102 c and a spaced apart opposed side wall 102 d, and further comprises a top wall 103. Accordingly, passage 104, which is defined by sidewalls 102 a, 102 b, 102 c and 102 d is rectangular in transverse section. In other embodiments, holding tank 100 may comprise, for example, a single rounded sidewall so as to have a transverse section that is circular, elliptical or the like. It will be appreciated that any other transverse section may be utilized.

Passage 104 is preferably longitudinally extending, for example along axis 105, and comprises an upper portion 106, and a lower portion 108. Passage 104 preferably extends vertically. However passage may extend generally vertically (i.e., at an angle to the vertical such that feedstock will flow downwardly therethrough under the force of gravity). In some embodiments, passage 104 may have a length along axis 105 of between about 5 ft and about 10 ft.

An inlet 110 is provided adjacent upper portion 106, and an outlet 112 is provided adjacent lower portion 108, at an elevation below the inlet 110. In the embodiment shown, inlet 110 is defined by an opening in top wall 104, and outlet 112 is defined by the lower ends 114 of sidewalls 102. It will be appreciated that inlet 110 may comprise the entirety of the top end of holding tank 100 and accordingly, a top surface 103 may not be required. It will be appreciated that in the preferred embodiment, no lower surface is provided for passage 104 and that the lower end of passage 104 is open. Accordingly, feedstock may flow downwardly through passage 104 unimpeded until it encounters feedstock stored in holding tank 100 or until it encounters discharge member 116. As exemplified, inlet 110 is in fluid communication with and receives cellulosic feedstock from outlet 18 of impregnation chamber 10, and outlet 112 is in fluid communication with and directs cellulosic feedstock to one or more auto hydrolysis reactors (not shown).

Referring still to FIG. 5, the lower end of passage 104 has a greater cross sectional area than upper end of passage 104. That is, a transverse cross section taken through passage 104 adjacent outlet 112 has a greater cross sectional area than a transverse section taken through passage 104 adjacent inlet 110. For example, the cross sectional area taken adjacent outlet 112 may have an area of between about 40 ft² and about 60 ft² and the cross sectional area taken adjacent inlet 110 may have an area of between about 20 ft² and about 40 ft².

Sidewalls 102 may be configured in a variety of ways in order to provide lower portion 108 with a greater cross sectional area than upper portion 106. In the embodiment shown, sidewall 102 a and sidewall 102 c are opposed to each other, and diverge from each other going from inlet 110 to outlet 112. Further sidewall 102 b and sidewall 102 d are opposed to each other, and diverge from each other going from inlet 110 to outlet 112. Accordingly, passage 104 is substantially frusto-pyramidal, and lower portion 108 has a greater cross sectional area than upper portion 104. In an alternate embodiment, sidewalls 102 a and 102 c may extend substantially parallel to each other, and sidewalls 102 b and 102 d may diverge from each other. In yet another alternate embodiment, holding tank 100 may comprise a single rounded sidewall defining a frustoconical passage 104. In yet another embodiment, sidewalls 102 may be stepped. It is preferred that sidewalls 102 continually diverge and that they continually diverge for the entire length of passage 104 as exemplified. Preferably, they diverge at an angle A from the vertical from about 1° to about 20°, preferably from about 2° to about 5°. It will also be appreciated that inner surface 138 of sidewalls 102 are preferably smooth and clear of projections that could be a source causing bridging to occur.

Providing lower portion 108 with a greater cross sectional area than upper portion 106 may aid in preventing cellulosic material from adhering or sticking to sidewalls 102 as the cellulosic material passes through holding tank 100. Accordingly, each portion of cellulosic feedstock that passes through holding tank 100 may have essentially the same residence time in passage 104.

In some embodiments, the feedstock may travel directly downwardly to the next process unit. In such a case, it is preferred the flow passage continually increase in cross sectional area (as opposed to using a hopper). However, it is preferred that the feedstock, after traveling downwardly through passage 104, is conveyed laterally (transverse to axis 105). Further, it is preferred that the feedstock is actively withdrawn from holding tank 104 instead of permitting the feedstock to passively flow out therefrom. Accordingly, in the exemplified embodiment, holding tank 100 is seated on a discharge member 116 adjacent outlet 112. Discharge member. 116 is configured to convey the celluslosic material laterally across outlet 112 to actively withdraw the cellulosic feedstock from holding tank 100.

Referring to FIGS. 5 to 8, in the embodiment shown, discharge member 116 comprises a base 118, sidewalls 120, and an open top 122. Open top 122 is preferably at least as large as outlet 112, and is in vertical registration with outlet 112, such that material passing through outlet 112 may pass directly downwardly through open top 122. It will be appreciated that in alternate embodiments, sidewalls 102 of passage 104 may provide the sidewalls of discharge member 116. That is, sidewalls 102 may extend beyond outlet 112. Accordingly, in such an embodiment, outlet 112 of passage 104 may not be defined by ends 114 of sidewalls 102, and rather, may be defined by a portion of sidewalls 102 above ends 114.

Discharge member 116 may use any transport mechanism known in the art to actively transport feedstock laterally from outlet 112. For example, a discharge member 116 may comprise an auger, a screw conveyor, drag line conveyor, paddle conveyor or the like that extends transversely to axis 105. Discharge member 116 comprises at least a first discharge member outlet 124, through which cellulosic feedstock exits discharge member 116. Cellulosic feedstock exiting discharge member outlet(s) 124 may pass into one or more conduits 125, which may, for example, lead to one or more, e.g., auto hydrolysis reactors (not shown). Preferably more then one outlet 124 is provided. An advantage of having more then one outlet is that two treated feedstock stream may be provided from holding tank 100.

As exemplified, discharge member 116 comprises two discharge member outlets 124 a, 124 b (see FIG. 7). Preferably, each outlet 124 is positioned such that it is not underneath passage 104 (laterally spaced from passage 104) and preferably more then one outlet 124 is provided. An advantage of positioning outlets 124 laterally from passage 104 is that feedstock may be withdrawn from all of outlet 112 and, more preferably, evenly from across outlet 112. Further, discharge member outlets 124 a and 124 b are preferably positioned on opposite sides of discharge member 116. Accordingly, discharge member outlets 124 a and 124 b may direct cellulosic material to two different, e.g., auto hydrolysis reactors, positioned on opposite sides of holding tank 100. As exemplified in FIGS. 1, 6 and 7, discharge member 116 may have upper wall 166 that extends over the top surface of discharge member 116 positioned laterally of holding tank 100. Top wall 166 may cover the portion of screw conveyor 126 positioned laterally of holding tank 100. Optionally, a grate 168, or other member that provides a window, may be positioned in top wall 166 above outlet 124. Grate 168 permits a worker to observe the travel of feedstock into conduits 125.

Discharge member 116 preferably comprises at least one screw conveyor 126 mounted above base 118. As exemplified, each screw conveyor 126 comprises a shaft 128 and at least one helical flighting 130 extending about the shaft, and is configured to rotate to engage material exiting outlet 112, and to convey it towards one of the discharge member outlets 124. Shaft 128 may be rotatably mounted by any means known in the art. As exemplified, shaft 128 has one end journalled in a bearing housing 164 and a second end journalled in a transmission housing 162. In the embodiment shown, discharge member 116 comprises a plurality of screw conveyors 126, which are arranged in pairs. First pair 132 a comprises screw conveyors 126 a and 126 b, which rotate about respective first 134 a and second 134 b generally parallel axes, and second pair 132 b comprises screw conveyors 126 c, and 126 d, which rotate about respective first 134 c and second 134 d generally parallel axes. Each of axes 134 are preferably horizontal, but may be at an angle of up to 45° or greater from the horizontal. Accordingly, screw conveyors 126 a and 126 b transport treated feedstock to outlet 124 a and screw conveyors 126 c and 126 d transport treated feedstock to outlet 124 b, which is on an opposed side to outlet 124 a. It will be appreciated that screw conveyors 126 a, 126 b, 126 c and 126 d extend under essentially all of outlet 112. Therefore, the screw conveyors 126 preferably withdraw treated feedstock for all portions of outlet 112. Alternately, or in addition, each outlet 124 may have one or more screw conveyors 126 or other transport member associated therewith.

Referring still to FIG. 7, as exemplified, screw conveyors 126 a and 126 b of first pair 132 a may each be rotated in a direction indicated by arrow Al, to feed material from above towards discharge member outlet 124 a. Further, screw conveyors 126 c and 126 d of second pair 132 b may each be rotated in a direction indicated by arrow A2, to feed material from above towards discharge member outlet 124 b.

In order to permit each screw conveyors 126 to be rotated, in a particular direction of rotation, each screw conveyor may be driven by its own drive motor 160. As shown in FIGS. 6 and 7, each shaft 128 extends outwardly past sidewall 120 into a transmission housing 162 wherein motor 160 is drivingly connected to shaft 128. Any driving linkage known in the art may be used. It will be appreciated that in an alternate embodiment, a single motor 160 may drive two or more shafts.

Accordingly, as exemplified, discharge member outlets 124 a and 124 b are positioned on laterally opposite sides of discharge member 116, and each helical flighting 130 is right-handed. Accordingly, direction A1 and direction A2 are opposite to each other. However, in alternate embodiments, discharge member outlets 124 a and 124 b may be positioned on the same lateral side as each other. In such an embodiment, directions A1 and A2 may be substantially the same. In yet further alternate embodiments, the helical flighting 130 of the first pair 132 a of screw conveyors 126 a, 126 b, may be right handed, and the helical flighting 130 of the second pair 132 b of screw conveyors 126 c, 126 d may be left handed. Accordingly, in such an embodiment, the first pair may rotate in the same direction as the second pair, and convey material in an opposite direction. It will be appreciated that each pair of screw conveyors 126 may be configured such that they rotate in opposite directions. For example, screw conveyor 126 a may be configured to rotate clockwise and screw conveyor 126 b may be configured to rotate counterclockwise.

Preferably, the helical flighting 130 of each screw conveyor has a first pitch adjacent its respective discharge member outlet 124 (i.e. the discharge member outlet towards which it conveys cellulosic feedstock), and a second pitch distal to its respective discharge member outlet 134 narrower than the first pitch. That is, screw conveyors 126 a and 126 b have a first pitch at end 158 adjacent discharge member outlet 124 a, and a second narrower pitch at the end 156 that is distal to discharge member outlet 124 a; and screw conveyors 126 c and 126 d have a first pitch adjacent discharge member outlet 124 b, and a second narrower pitch distal to discharge member outlet 124 b.

In the embodiments shown, the pitch of each helical flighting 130 gradually becomes wider towards each discharge member outlet 124. For example, the pitch may vary gradually from between about 4 inches and about 8 inches at the distal end 156 of screw conveyor 126 to between about 14 inches and about 16 inches the end 158 of screw conveyor 126 adjacent discharge member outlet 124. In alternate embodiments, an abrupt transition between wider and narrower regions of flighting may occur. For example, each screw conveyor may comprise a first flight adjacent a discharge member outlet 124 and a second flight upstream from the first flight. The first flight may have a first constant pitch, for example of between about 14 inches and about 18 inches, and the second fight may have a second constant pitch narrower than the first constant pitch, for example of between about 4 inches and about 8 inches. In a further embodiment, a third intermediate flight having a third constant pitch wider than the second flight and narrower than the first flight may be positioned between the first flight and the second flight. The third flight may have a pitch of between about 6 inches and about 10 inches, for example.

Providing each screw conveyor with a narrower pitch distal to the discharge member outlet may allow for substantially equally amounts of material to be withdrawn from each portion of outlet 112. That is, material deposited in screw conveyor 126 at the distal end 156 will be conveyed towards the respective outlet 124 for that screw conveyor. As that material is transported laterally, the pitch of the screw increases permitting additional material to be deposited directly in the screw conveyor from outlet 112. Further increases in the pitch will permit additional portions of the material to fall into screw conveyor. The portion or portions of the screw conveyor closer to outlet 124 (in the direction of transport) have a wider pitch such that it may accommodate material conveyed from the distal region, as well as material deposited directly thereon from passage 104. Accordingly, feedstock is withdrawn from across all of outlet 112.

It will be appreciated that in alternate embodiments, one or more screw conveyors 126 may be otherwise configured. For example, discharge member 116 may comprise only one screw conveyor 126 and one outlet 124, or discharge member 116 may comprise a plurality of screw conveyors which are not arranged in pairs, or discharge member 116 may comprise more than two pairs of screw conveyors. For example, in an alternate embodiment shown in FIG. 9, discharge member 116 comprises four discharge member outlets 124, and four pairs 132 of screw conveyors 126.

Referring to FIGS. 5 and 10, holding tank 100 preferably further comprises a heating jacket 136 provided on at least a portion of the sidewalls 102. For example, in the embodiment shown, heating jacket 136 surrounds all of each sidewall 102. Heating jacket 136 comprises a plurality of outer walls 140 that are generally parallel to and spaced from sidewalls 102 so as to define an enclosure 142 therebetween. A fluid may be passed through enclosure 142 from an inlet (not shown) to an outlet (not shown) so that a heated fluid is through enclosure 142. Heating jacket 136 may be of any construction known in the art. Accordingly, the cellulosic material may be heated to a desired temperature, or maintained at a desired temperature as it passes through holding tank 100.

Referring to FIG. 5, in a further preferred embodiment, discharge member 116 also comprises a heating jacket 146 provided on sidewalls 120 and/or base 118 of discharge member 116. Heating jacket 146 may be configured similarly to heating jacket 136, and may comprise an outer wall 154 spaced outwardly from sidewalls 120 and/or base 118 and is configured for passing a heated fluid through an enclosure 150 defined between outer walls 154 and sidewalls 120 and/or base 118.

A method of treating a cellulosic feedstock for ethanol production will now be described. Although the method will be described with reference to holding tank 100, it will be appreciated that the method may be carried out using an alternate apparatus, and holding tank 100 may be operated according to an alternate method.

A suitable cellulosic feedstock is preferably first treated to moisture impregnation to raise the moisture content of the feedstock to a predetermined level. Preferably, the moisture content of the feedstock upon entry to the holding tank is from about 30wt % to about 60wt %, preferably from about 45wt % to about 55wt %. This may be achieved by passing the feedstock through an impregnation chamber to an outlet of the impregnation chamber. The impregnation chamber may be, for example, impregnation chamber 10, and may comprise one or more conveyance members for urging the cellulosic feedstock along the impregnation chamber towards outlet 18 of impregnation chamber. As the cellulosic feedstock is passed through the impregnation chamber, it may be pretreated by one or more of moistening the cellulosic feedstock and heating the cellulosic feedstock, as described in U.S. Publication Number 20100028089 A1, the disclosure of which is incorporated herein by reference in its entirety.

The cellulosic feedstock with or without being subjected to impregnation, is then conveyed to a holding tank wherein the feedstock is conveyed downwardly and laterally as it travels through the holding tank. For example, referring to holding tank 100, inlet 110 is disposed at an elevation above outlet 112. Accordingly, the material may migrate downwardly from the inlet towards the outlet under the force of gravity. Furthermore, as lower portion 108 has a greater cross sectional area than upper portion 106, the material will migrate laterally as it migrates downwardly.

Preferably, the method further comprises laterally conveying the cellulosic feedstock from the passage through the holding tank. Accordingly, once the feedstock reaches the exit of the holding tank, the feedstock is conveyed laterally to, e.g., one or more conduits in flow communication with a downstream process unit. For example, the holding tank may comprise a discharge member, such as discharge member 116, adjacent lower end 108. One or more screw conveyors 126 of the discharge member may convey the cellulosic feedstock laterally across outlet 112, as described hereinabove.

Alternately, or in addition, the cellulosic material is preferably actively withdrawn from essentially the entirety of outlet 112. The feedstock is therefore moved at least with an assistance of machinery out of the holding tank. For example, discharge member 116 may comprise a plurality of screw conveyors extending across outlet 112, which, when rotated, engage the cellulosic material adjacent the entirety of the outlet 112, and convey it towards a discharge member outlet, as described hereinabove.

In any embodiment, generally equally amounts of the cellulosic material is preferably withdrawn from each portion of the outlet 112. For example, a screw conveyor 126 of a discharge member 116 may have a helical flighting having a first pitch adjacent a discharge member outlet, and second pitch narrower than the first pitch distal to the discharge member outlet. Accordingly a generally equal amount of feedstock is withdrawn from the region adjacent a discharge member outlet, and from a region distal to a discharge member outlet.

In some embodiments, a first portion of the cellulosic feedstock is preferably withdrawn in a first lateral direction and a second portion of the cellulosic feedstock withdrawing a second portion of the cellulosic feedstock is preferably in a second lateral direction, which is preferably opposite to the first direction. For example, the holding tank may comprise a discharge member having a first pair of screw conveyors and a second pair of screw conveyors. Each screw conveyor may comprise a right-handed helical flighting, and the first pair of screw conveyors may be rotated in a first direction to convey the cellulosic feedstock in a first lateral direction, and the second pair of screw conveyors may be rotated in a second direction to convey the cellulosic feedstock in a second lateral direction.

In some embodiments, the method preferably further comprises maintaining a temperature in the passage of the holding tank between about 50° C. and about 75° C. For example, the holding tank may be provided with a heating jacket, such as heating jacket 136. The heating jacket may serve to heat the walls of the holding tank, such that the material within the holding tank is maintained at or raised to a temperature between 50° C. and about 75° C.

The method preferably comprises operating the holding tank such that the cellulosic feedstock moves from the inlet to the outlet in about 10 to 30 minutes. Accordingly, in use, the method may be preceded by an initial start up phase, wherein a discharge member of the holding tank is not operated, and the tank is filled with cellulosic feedstock from impregnation chamber 12. When the tank is filled, the method may commence, such that the holding tank is operated at steady state with a predetermined residence time.

It will be appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments or separate aspects, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment or aspect, may also be provided separately or in any suitable sub-combination.

Although the invention has been described in conjunction with specific embodiments thereof, if is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. 

We claim:
 1. A holding tank for a cellulosic feedstock comprising: at least one sidewall defining a passage having an upper portion and a lower portion, the lower portion having a greater cross sectional area than the upper portion; at least one inlet adjacent the upper portion; at least one outlet adjacent the lower portion, at an elevation below the inlet; and at least one discharge member adjacent the at least one outlet wherein the at least one discharge member comprises a base, an open top that is at least as large as, and is in vertical registration with, the at least one outlet, and at least a first discharge member outlet that is laterally positioned and conveys the cellulosic feedstock laterally across the outlet, wherein (i) the at least one discharge member comprises a first pair of screw conveyors, each conveyor in the first pair rotating about respective first and second generally parallel axes, (ii) the screw conveyors have a first pitch adjacent the first discharge member outlet, and a second pitch narrower than the first pitch distal to the first discharge member outlet, (iii) the axes are generally horizontal and (iv) the cellulosic feedstock is essentially a solid.
 2. The holding tank of claim 1, wherein the sidewalls comprise a first sidewall and a second sidewall opposed to the first sidewall, the first and second sidewalls diverging relative to each other from the upper portion to the lower portion.
 3. The holding tank of claim 2, wherein the sidewalls comprise a third sidewall and a fourth sidewall opposed to the third sidewall, the third and fourth sidewalls extending between the first and second sidewalls, the third and fourth sidewalls diverging relative to each other from the upper portion to the lower portion.
 4. The holding tank of claim 1, wherein the at least one discharge member comprises at least one screw conveyor mounted above the base.
 5. The holding tank of claim 1, wherein the screw conveyors are rotated in the same direction and feed material from above towards the first discharge member outlet.
 6. The holding tank of claim 1 wherein the screw conveyors each have a first flight and a second flight, the first flight having the first pitch and the second flight the second pitch.
 7. The holding tank of claim 6, wherein each screw conveyor comprises a third flight intermediate the first and second flights, the third flight having an intermediate pitch that is wider than the first pitch and narrower than the wider pitch.
 8. The holding tank of claim 1, wherein the discharge member further comprises a second pair of screw conveyors.
 9. The holding tank of claim 8, wherein the second pair of screw conveyors urges material from above towards a second discharge member outlet that is laterally positioned and spaced away from the first discharge member outlet.
 10. The holding tank of claim 8, wherein the second discharge member outlet is in a different direction to the first discharge member outlet.
 11. The holding tank of claim 8, wherein second pair of screw conveyors are rotatable about parallel axes.
 12. The holding tank of claim 11, wherein each screw conveyor in the second pair rotates in the same direction.
 13. The holding tank of claim 12, wherein the screw conveyors of first pair rotate opposite to the screw conveyors of second pair.
 14. The holding tank of claim 11, wherein the axes are generally horizontal.
 15. The holding tank of claim 1, further comprising a heating jacket provided on at least a portion of the at least one sidewall.
 16. The holding tank of claim 1, further comprising a heating jacket provided on the at least one sidewall. 